Vegans and vegetarians (once a month meat consumption isnt really an omnivore diet) are more likely to be deficient in Zinc, Iron and are more susceptible to osteoporosis due to poor Calcium uptake. Because animal protein does help the body to absorb minerals like Iron, Zinc and the like (it isnt known for sure why and phytates in certain plant foods also hampers mineral uptake) vegetarians and vegans need to overcompensate for those minerals in their food. On the order of about 50% higher than the RDA for omnivores.

Now I am not saying it cant be healthy, it can and there certainly are problems with how the average westerner eats, but I have no confidence in this being done correctly on a mass scale given the data that has come out. eg. 50% of vegans are deficient in B12 as measured by blood test and thats among a population that is likely much more aware of B12 being problematic since it is only naturally found in significant quantities in animal products and almost every meat and dairy substitute on the market is fortified with B12. And that widespread deficiency STILL happens. Vegetarians are less susceptible to B12 deficiency but still generally rely on the dairy industry to obtain that B12 along with Calcium and Zinc. And because B12 is water soluble not fat soluble, it needs to be obtained daily or in higher doses, semidaily. And the effects of B12 deficiency can be delayed months (pernicious anemia) or years (permanent nerve damage with the anemia hidden by excess folate consumption)

People need education and better meat and dairy substitutes that arent as processed to make this work. Right now, most of them have too much salt and saturated fats to be an improvement.

You need cows to produce all of that just like you would meat. The way cow physiology works requires that calves be birthed periodically to maintain milk production at large scale. The dairy industry is to a large extent, a by product of the meat industry. Those new calves have to go somewhere. And you have to keep in mind that 70% of the world's population is lactose intolerant as adults. They rely on nondairy meat products for the majority of the B12 they get. OR you switch people to vegan substitutes that have B12 added to them. Right now those are niche/luxury products which is problematic for developing nations. Like... imagine going from small scale cattle and poultry farming to relying on B12 bacterial fermenters and soy production at large scale. That might be doable if new processes for using certain strains of B12 producing pseudomonas bacterial cultures can be developed for fermented soy products like tempeh can spread there but again, those arent there yet. More R&D is needed.